[cmake] Make libcxx(abi) a dependency when building in-tree clang for macOS

[JDevlieghere]

Summary

As discussed on IRC this morning, when building an in-tree clang for testing we have to have libcxx and libcxxabi checked out. This is a common pitfall that manifests that is non-obvious and manifests itself only after building when running the test suite.

[mgorny]

How does this affect platforms that use other ABI library than libcxxabi? How does this affect platforms that use libstdc++ as default C++ backend?

[JDevlieghere]

In D55837#1334872, @mgorny wrote:

How does this affect platforms that use other ABI library than libcxxabi? How does this affect platforms that use libstdc++ as default C++ backend?

I'm not familiar with that use case myself, so I might be able to give a better answer if I understand how that works today. I think it wouldn't change anything other than that the target has to be present.

[labath]

On linux libc++ is not a hard dependency for running tests (in fact, clang will not use it even if it is checked out). I guess that has something to do with the fact that libc++ is not the default c++ implementation on that platform. I think this will need to be darwin-only or something like that.

[JDevlieghere]

Only do this for macOS

[JDevlieghere]

In D55837#1334893, @labath wrote:

On linux libc++ is not a hard dependency for running tests (in fact, clang will not use it even if it is checked out). I guess that has something to do with the fact that libc++ is not the default c++ implementation on that platform. I think this will need to be darwin-only or something like that.

I didn't know, cool, then that solves both issues.

[clayborg]

CMakeLists.txt

160

What if the user wants to use a different compiler for tests? cmake has LLDB_TEST_USE_CUSTOM_C_COMPILER and LLDB_TEST_C_COMPILER and LLDB_TEST_CXX_COMPILER which can supposedly be set.

[JDevlieghere]

CMakeLists.txt

160

That still works. The change says that *if* you are going to build the in-tree clang, it has to build libcxx(abi) too. That should always make sense, because clang (on Darwin) won't be able to find the standard library otherwise.

CMakeLists.txt

if(LLDB_INCLUDE_TESTS)
   endif()
 
   if(TARGET liblldb)
     list(APPEND LLDB_TEST_DEPS liblldb)
   endif()
 
   if(TARGET clang)
     list(APPEND LLDB_TEST_DEPS clang)
+    if(APPLE)
+      list(APPEND LLDB_TEST_DEPS cxx)
+      list(APPEND LLDB_TEST_DEPS cxxabi)
+    endif()
   endif()
 
   if(TARGET dsymutil)
     list(APPEND LLDB_TEST_DEPS dsymutil)
   endif()
 
   add_subdirectory(test)
   add_subdirectory(unittests)